Method for hydraulic fracturing of a hydrocarbon formation

ABSTRACT

A fracturing fluid is injected under a high pressure into a well drilled in a formation to create a hydraulic fracture. Then a suspension of the hydraulic fracturing fluid mixed with proppant particles is injected into the well and the created hydraulic fracture, the suspension having a consistency coefficient greater than 0.1 Pa s n  at any flow index n and a yield stress higher than 5 Pa. Then, an overflush fluid having a consistency coefficient lower than 0.01 Pa·s n  is injected into the well.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Russian Application No. 2016140355filed Oct. 13, 2016, which is incorporated herein by reference in itsentirety.

BACKGROUND

The disclosure relates to oil and gas industry and can be used toincrease the productivity of both newly introduced and operatingproduction and injection wells.

The methods for stimulating oil or gas production by means of hydraulicfracturing of a hydrocarbon formation is widely known. Typically, afracture is created by injecting a clean hydraulic fracturing fluidunder a high pressure through a well into the rock. The open fracture isthen filled with a suspension of the fluid mixed with sand (proppantparticles), which then keeps the fracture open. Finally, a small amountof a clean fluid is injected into the well to clean the wellbore fromsolid particles, and a part of this fluid can go into the fracture. Thislast step is called injection of an overflush fluid.

The practice of injecting an overflush fluid, i.e., displacement of ahydraulic fracturing suspension from a wellbore into the fracture by alow-viscosity fluid, is usually applied at completion of horizontalwells drilled in formations of unconventional gas by a method ofmulti-stage hydraulic fracturing. It provides purification from solidparticles (proppant) for subsequent operations or steps and prevents theflowback of solid particles from the fracture when a well is started.However, the injection of the overflush fluid can negatively affect theoverall production of the fractures due to a combination of factors.First, the proppant can be displaced quite far from the well into thefracture, so that the fracture will be unsupported near the well and canclose there at any moment of the operation time of the well, when thefluid pressure is not enough to keep the fracture open (from thebeginning of the production till the next moment in the process ofoperating the well). Furthermore, after stopping the injection, thefracture is closed long enough in low permeable reservoirs (the fluidoutflow into the rock through the walls of the fracture takes a longtime). In the process of closing the fracture, the suspension can swellto the bottom of the fracture by gravity, while the pure fluid risesupward, leaving a significant part of the near-field area withoutsupport and blocking the access to the top of the reservoir.

Various methods for improving operation of hydraulic fracturing withinjection of an overflush fluid are known from the prior art. Thus, U.S.Pat. No. 7,104,325 proposes sealing a phase injected prior to injectionof the overflush fluid by adding a proppant coated with resin.

In U.S. Pat. No. 3,752,233 the addition of hydrazine to a phase of anoverflush fluid is provided to restore permeability, wherein thepermeability is adversely affected by the high molecular weight of apolymer in a fracturing fluid.

U.S. Pat. No. 2,859,819 discloses a last phase of a pure low-viscosityfluid (overflush fluid) to remove particles from a wellbore into thefracture for cleaning the well.

In all known methods, the risk of closing the fracture remains due tothe possibility of creating an unfixed region in the near-field area ofthe fracture.

SUMMARY

The technical result achieved in implementation of the disclosureincludes significant reduction of the risk of fracture closure and lossof hydraulic connection between a well and a fracture due to thereduction of a proppant particle-free unfixed region in a near-fieldarea of the fracture.

In accordance with the proposed method, a fracturing fluid is injectedunder a high pressure into a well drilled in a formation to create ahydraulic fracture. Then a suspension of the hydraulic fracturing fluidmixed with proppant particles is injected into the well and the createdhydraulic fracture, the suspension having a consistency coefficientgreater than 0.1 Pa s^(n) at any flow index n and a yield stress higherthan 5 Pa. An overflush fluid having a consistency coefficient lowerthan 0.01 Pa·s^(n) is then injected into the well.

In accordance with an embodiment of the disclosure, the overflush fluidcomprises a chemical breaker capable of reacting with the suspensionfluid to transform the suspension into a power linear gel without yieldstress.

Optimal parameters of the suspension and the overflush fluid aredetermined using numerical simulation of the overflush fluid injectionoperation.

In accordance with a further embodiment of the disclosure, a diameter ofthe proppant particle-free region created by the overflush fluid isadjusted in the near-field area within the fracture by adjusting aninjection rate of the overflush fluid. The overflush fluid is injectedat a rate higher than a threshold rate u_(c) at the suspension—overflushfluid interface when a behavior factor of the overflush fluid is greaterthan that of the fracturing fluid, or at a rate lower than the thresholdrate u_(c) at the suspension—overflush fluid interface when the behaviorfactor of the overflush fluid is lower than that of the fracturingfluid.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure is illustrated by the drawings, where

FIG. 1 is a schematic layout of a horizontal well intersecting atransverse hydraulic fracture, with an overflush fluid displacing asuspension;

FIG. 2 shows simulation results for displacement of the suspension bythe overflush fluid in the right half of the fracture; and

FIG. 3 shows the relationship of effective viscosities of the fluidsversus a local linear velocity in the fracture.

DETAILED DESCRIPTION

The disclosure is aimed at optimizing an area of that portion of thefracture that may remain unfixed before significant closure of thefracture occurs when the pressure in the formation decreases. Thedisclosure provides a reliable limit for the volumes of an overflushfluid. The disclosure is based on mathematical simulation of thisprocess and the parametric study of various strategies for the overflushfluid injection, with attention being paid to displacing a suspension bythe overflush fluid, changing volume and injection velocity of theoverflush fluid, and the rheological contrast between the suspension andthe overflush fluid. The shape of a proppant particle-free region can beregulated by the injection rate of the overflush fluid based on theSeffman-Taylor instability criteria applied to the phase interfacebetween the overflush fluid and the suspension with the proppantparticles. It was found that (i) when “fingers” of the overflush fluidare formed at the overflush fluid—suspension interface in the near-fieldarea, large suspension pillars can be maintained that can keep thefracture open; (ii) the swelling of the suspension with the proppant canbe reduced by means of rheology with a yield stress for the suspensionand/or high viscosity of the base fluid used for preparation of thesuspension (viscosity of the base fluid is characterized by theconsistency index for the case of power rheology); (iii) for a certaindistance from perforations, there is a threshold injection rate(calculated on the basis of the rheological properties of the fluid),which determines the instability of the interface between the overflushfluid and the fracturing fluids.

The hydraulic fracturing job in a sub horizontal well drilled in anunconventional gas or oil bearing formation, has been optimized.

This optimization focuses on a final stage of the hydraulic fracturingjob; when after the injection of the fluid suspension with the particlesfor propping the fractures (proppant), a small amount of clean fluid isintroduced to clean the well from the proppant particles and to displaceall particles into the fractures (the “injection of an overflush fluid”stage).

The properties of the suspension and the overflush fluid are adjusted sothat the proppant-free region in the near-wellbore zone of the fractureis minimized thereby decreasing the risk of closure and loss of thehydraulic connection between the well and the fracture.

More specifically, the suspension injected immediately before the stageof injection of the overflush fluid shall have rheological propertiesaccording to the Herschel-Balkley model (shear dilution in combinationwith the yield stress), and the consistency coefficient shall be greaterthan 0.1 Pa^(n) s^(n), and the yield stress—greater than 5 Pa.

Within the specified range of properties, the displacement of thesuspension by a clean “overflush” fluid causes development of small“fingers” of the overflush fluid penetrating into the suspension (unlikethe large “islands” of the clean fluid when the suspension has a lowerconsistency or yield stress) thereby decreasing an area unsupported bythe proppant, and minimizing the risk of fracture closing in thenear-wellbore zone (see FIG. 1). FIG. 2 shows simulation results fordisplacing the suspension by the overflush fluid in the right half ofthe fracture, where an unfixed cavity in the immediate vicinity of thewell is shown on the left side as a semicircle in gray.

The overflush fluid can contain an admixture of chemicals which act as a“breaker” for a crosslinked gel containing particles (suspension).Because of the reaction of this breaker with the suspension, thecrosslinks between the polymer molecules in the carrier fluid of thesuspension are broken, the yield stress disappears, and the particlesmove from the suspension to the fingers of the overflush fluid.Therefore, the fingers of the overflush fluid, that initially were notsupported, are filled finally with some amount of proppant, and the riskof closing the fracture inside these fingers is reduced. Oxidants,enzymes or acids can be used as diluents.

The exact values of the properties of the suspension and the overflushfluid in the specified range can be determined by numerical simulationof the injection operation of the overflush fluid. Numerical simulationis based on implementation of a mathematical model for multiphase flowin a fracture. The optimal properties can be obtained from the analysisof the unfixed portions calculated during the simulation for differentvalues of the properties of the suspension and the overflush fluidwithin the specified range.

When the rheology of fracturing fluids and an overflush fluid is a powerlaw fluid, or when the overflush fluid is a Newtonian fluid (forexample, water) and the fracturing fluid is a power law fluid, there isa threshold of linear velocity inside the fracture that defines thefront instability during the injection of the overflush fluid.Consequently, it is possible to control the shape of the injectionregion of the overflush fluid within the fracture. In particular, it ispossible to initiate the instability and development of viscous fingersat a certain distance from the wellbore, thus providing the followingeffects: (i) reduction of proppant flow to the well by fully displacingthe hydraulic fracturing fluid in a certain (small) region of thehydraulic fracture near perforations; and (ii) initiation of instabilityat the interface between the overflush fluid and hydraulic fracturingfluid at a certain distance from the perforations to cause thedevelopment of proppant columns and to reduce the development of anexcessively unfixed (proppant-free) region.

The control of the displacement process and the shape of the region withthe overflush fluid within the hydraulic fracture is described by adifference in the effective viscosities of the fluids in the formation,which depend on the local linear velocity inside the fracture. This willbe described in more details below.

At the first stage, a hydraulic fracturing fluid is injected under ahigh pressure into a well drilled in a formation. Linear or crosslinkedgels with density of 1000 kg/m³ and viscosity of 0.01 Pa·s for a lineargel or 0.1 Pa·s for a crosslinked gel can be used as the hydraulicfracturing fluid. For example, the use can be made of water-based lineargels and crosslinked gels that are obtained from the linear gels byadding a crosslinker, for example, based on borate. An example of a gelused is an aqueous gel having the following composition: 1 L of water,20 g of KCl, 4 g of guar, 0.14 g of boric acid and 0.14 g of sodiumhydroxide.

At the next stage, the hydraulic fracturing fluid suspension mixed withproppant particles is injected into the well and the created hydraulicfracture, and then—an overflush fluid. Fresh or formation water can beused as the overflush fluid.

Let a first fluid be an overflush fluid determined by a power rheologywith a consistency coefficient K₁ and a flow index n₁, and a secondfluid be a hydraulic fracturing fluid with a rheology determined by theparameters K₂ and n₂. The effective viscosity of the fluid flowingthrough hydraulic fracture is defined as follows:

$\begin{matrix}{\mu_{eff} = {{K( \frac{1 + {2n}}{3n} )}^{n}{\overset{.}{\gamma}}^{n - 1}}} & (1)\end{matrix}$

This expression is derived from the relationship between thewidth-averaged velocity inside hydraulic fracture and the pressuregradient in proximity of the thin layer (the “lubricationapproximation”) for the 3D Navier-Stokes equations. Here, {dot over(y)}=6u/w is the local (width-average of the fracture) shear velocity,involving the local width-average velocity u and the fracture width w.The parameters K and n are the consistency coefficient and the flowindex.

Consider the displacement of the second fluid, which fills the hydraulicfracture, with the first fluid entering the transverse fracture throughthe perforations. Near the perforations the flow is radial, so the massconservation equation gives the following expression for the velocity:

$\begin{matrix}{{u(r)} = \frac{u_{0}r_{0}}{r}} & (2)\end{matrix}$

Here, r₀ is a radius of the well, r is a distance between the well axisand the specific location inside the fracture, u₀ is a velocity inperforations (for simplicity, assume that the perforations aredistributed evenly along the well casing string).

The instability at the interface between these fluids occurs when thelocal effective viscosity of the first fluid is less than the localeffective viscosity of the second fluid. This condition can be expressedas follows:

$\begin{matrix}{{{K_{1}( \frac{1 + {2n_{1}}}{3n_{1}} )}^{n_{1}}{\overset{.}{\gamma}}^{n_{1} - 1}} < {{K_{2}( \frac{1 + {2n_{2}}}{3n_{2}} )}^{n_{2}}{\overset{.}{\gamma}}^{n_{2} - 1}}} & (3)\end{matrix}$

Inequality (3) gives the following threshold velocity u_(c) at theinterface:

$\begin{matrix}{u_{c} = {\frac{w}{6}( \frac{K_{2}}{K_{1}} )^{1/{({n_{1} - n_{2}})}}( \frac{1 + {2n_{1}}}{3n_{1}} )^{{- n_{1}}/{({n_{1} - n_{2}})}}( \frac{1 + {2n_{2}}}{3n_{2}} )^{n_{2}/{({n_{1} - n_{2}})}}}} & (4)\end{matrix}$

So that if n₁>n₂:μ_(eff,1)<μ_(eff,2)⇔u<u_(c), and ifn₁<n₂:μ_(eff,1)<μ_(eff,2)⇔u>u_(c).

In particular, if the first fluid is Newtonian (μ_(eff,1)=K₁=μ, n₁=1),and the second is not Newtonian (power law), then the instabilitycriterion is formulated as follows (0<n₂<1):

$\begin{matrix}{{u < u_{c}} = {\frac{w}{6}( \frac{K_{2}}{\mu} )^{1/{({1 - n_{2}})}}( \frac{1 + {2n_{2}}}{3n_{2}} )^{n_{2}/{({1 - n_{2}})}}}} & (5)\end{matrix}$

At a constant injection rate, the linear velocity decreases inverselyproportional to the distance to the well axis (see equation (2)).Consequently, the equations (5) (and (4) for the case n₁>n₂) and (2) canbe combined to calculate the velocity in the perforations u₀, whichdetermines the occurrence of instability at a certain distance to theaxis R of the well. Alternatively, if one specifies the outer radius ofthe proppant particle-free cavity R, one can find the velocity in theperforations u₀ (which is related to the injection rate) necessary tocreate this cavity. In both cases the following relation is used:

$\begin{matrix}{u_{c} = \frac{u_{0}r_{0}}{R}} & (6)\end{matrix}$

Below is the relationship between the effective viscosities of the firstfluid and the second fluid for different fluids (see Table 1) and thecalculated linear velocity thresholds (FIG. 3). Further (for the tableand FIG. 3), it should be explained that the crosslinked gel and thelinear gel are a suspension with proppant, while water is the overflushfluid.

TABLE 1 Rheological parameters of the fluids considered as an exampleConsistency coefficient K Flow index n (Pa · s^(n)) (dimensionless)Water 0.001 1 Linear gel 1 0.2 0.4 Linear gel 2 0.1 0.5 Crosslinked gel1.8 0.35

Referring to FIG. 3, curve 1 denotes water displacing the linear gel 1;curve 2 is water displacing linear gel 2; curve 3 is a linear gel 1displacing the crosslinked gel. The rheological parameters of thesefluids are given in Table 1. The critical velocity u_(c), whichdetermines the beginning of formation of viscous fingers, is determinedfrom μ₁/μ₂=1.

1. A method for hydraulic fracturing of a hydrocarbon formation,comprising: injecting under a high pressure a hydraulic fracturing fluidinto a well drilled in the formation to create a hydraulic fracture;injecting a suspension of the hydraulic fracturing fluid mixed withproppant particles into the well and the created hydraulic fracture, thesuspension having a consistency coefficient greater than 0.1 Pa·s^(n) atany flow index n and a yield stress higher than 5 Pa; and injecting anoverflush fluid with a consistency coefficient lower than 0.01 Pa·s^(n)into the well.
 2. The method of claim 1, wherein the overflush fluidcomprises a chemical diluent capable of reacting with the suspension totransform the suspension into a power linear gel without yield stress.3. The method of claim 1, wherein optimal parameters of the suspensionand the overflush fluid are determined based on numerical simulation ofthe overflush fluid injection operation.
 4. The method of claim 1,wherein a diameter of a proppant particles-free region created by theoverflush fluid in a near-wellbore area within the fracture is adjustedby adjusting an injection rate of the overflush fluid.
 5. The method ofclaim 4, wherein the injection rate of the overflush fluid is higherthan a threshold rate u_(c) at a suspension—overflush fluid interfacewhen a behavior factor of the overflush fluid is greater than that ofthe hydraulic fracturing fluid, or the injection rate of the overflushfluid is lower than the threshold rate u_(c) at the suspension—overflushfluid interface when the behavior factor of the overflush fluid is lowerthan that of the hydraulic fracturing fluid.